The present exemplary embodiments relate to a method for preprinting and/or re-liquifying subpixels to achieve color uniformity in color filters. It finds particular application in conjunction with formation of color filters that are used in various displays such as liquid crystal displays and the like, and will be described with particular reference thereto. However, it is to be appreciated that the present exemplary embodiments are also amenable to other like applications.
By way of background, formation of color filters has been accomplished in a number of different manners. In one traditional technique, a black matrix defining pixels and subpixels for each of the color filter elements is formed on a glass substrate. Then, the glass substrate and the black matrix is appropriately masked so that a first color (e.g., green) can be applied to various subpixel areas. After the first color is applied, the mask is removed and the substrate and black matrix are masked in a different manner to accommodate a second color (e.g., blue) to be printed. After the second color is applied, the mask is removed and a third mask is applied to the substrate and matrix so that a third color (e.g., red) can be formed on the color filter.
The obvious drawbacks to this approach are that it is costly and time consuming. Use of multiple masks makes it difficult to efficiently process and form a color filter according to these techniques. Other similar processes for forming color filters have been used, but these other techniques generally require many iterations, such as the ones described above.
Formation of color filters using ink droplet printing techniques has become popular recently. However, the ink droplet printing techniques result in a variety of different problems. For example, if color filter subpixels that are printed using inkjet techniques do not dry uniformly, e.g. if they have dye or colorant distributed unevenly across the subpixel, then the transmitted color intensity may be also be non-uniform.
In addition, if it is desired to add or subtract drop quantity from a subpixel as they are being printed (as oftentimes occur), it is difficult to evenly distribute the modified number of drops across the subpixel so the colorant is uniform. For example, if 20 drops are normally printed evenly across the pixel, but the process is adjusted to 18, a new firing pitch is required to spread the drops out evenly. If the drops dry too quickly, they may not spread out properly and the drop size (or marked spot) might need to be adjusted. If these adjustments are not made, non-uniformities and scallops along the edge of the subpixel may occur.
Further, the surface wetting properties between the colored liquids and the boundaries of the subpixel (the black matrix) may not be sufficient to keep the liquid from spreading into neighboring subpixels. This can be a problem since mixing the red with the green would be catastrophic in terms of producing a useful color filter.
To further illustrate with reference to FIG. 1, a full pixel 10 having RGB subpixels 12 is shown. It should be understood that the full pixel 10 is typically repeated many hundreds or thousands of times across a suitably sized glass substrate that is ultimately used in the manufacture of displays such as liquid crystal displays. It should be further understood that the subpixels will typically be of red, green and blue color, respectively. A black matrix 14 frames each subpixel 12. The black matrix not only serves as a border element between colored subpixels 12, but also serves to provide contrast to the images that will ultimately displayed on the display. This partially completed pixel 10 shows how spots from droplet printing might create the subpixels. It can be seen that if the droplets, such as that shown at 16, do not cover the entire substrate, bright spots or scallops 22 occur. Looking at two red droplets 16 and 18 in an exploded view, coffee staining 20 can be seen in the radial distribution of the drops. Coffee staining is a drying phenomenon that occurs and typically results in lighter or darker coloring around the edges as shown by the reference numeral 20 as well as slight discoloration in the center of the printhead droplet. This staining is caused by non-uniform colorant distribution during drying. Of course, this causes non-uniform color in the subpixel.